Modern datacenters typically house hundreds or even thousands of servers. Each server is a computer system that may support one or more tenants. A server that supports multiple tenants typically runs multiple virtual machines (“VMs”) for these tenants. A VM is a software implementation of a computer. From the perspective of a tenant who has a VM residing in a server, the VM behaves as if it were a physical computer. The VMs communicate with each other by electronic messages. A VM in which a message originates hands one or more packets containing the message over to a communication facility in the server in which the originating VM resides. The communication facility, which typically may be a virtual switch, sends the packets to the server in which the destination VM resides. Traffic through the communication network must be controlled to ensure appropriate levels of service for all tenants. Congestion control protocols such as Transmission Control Protocol (TCP) provide this control by delaying packet transmission or by selectively dropping packets at arbitrary locations along network paths if there are too many packets for the available network bandwidth. An originating server responds to delays and dropped packets by slowing down its transmission rate.